The present invention relates generally to magnetic resonance (MR) imaging systems, and more particularly, to an apparatus and system for isolating mechanical vibrational disturbances between an MR imaging system and a surrounding structure and to a method of tuning the apparatus.
Magnetic resonance imagers employ electrically excited coils to impose time varying magnetic field current on a larger but steady primary field provided by a main magnet. The imposed currents exist within a magnetic field causing generation of corresponding forces on conductors contained therein. The corresponding forces result in generation of undesirable dynamic motions of the conductors that propagate throughout the magnetic resonance imager.
The dynamic motions of the conductors have associated waveforms that contain repetitive pulses with fast transitions. The waveforms impart vibrational energy within an audio frequency range and in turn radiate sound pressure waves at solid to air interfaces resulting in pronounced acoustic noise. The generated vibrations transmit through a cryostat of the magnetic resonance imaging system into a building structure, such as a floor, a wall, or other structure within a building environment. Upon being transferred into the building structure the vibrations may then propagate throughout a building site where they can emerge as noise sources in various areas of the building. The generated acoustical noise can cause irritation and interference with clinical procedures, such as for example listening for irregularities in a patient""s heart. In addition, high-speed scans in high field magnets can generate increased noise levels that are beyond acceptable exposure levels.
Although, a cryostat is typically rigidly fixed to a building structure several classical isolation methods have been attempted for isolation of the cryostat. The prior attempts have included use of coil springs, air shocks, and soft elastomeric isolation material pads in order to attenuate the vibrations and thus the acoustical noise. The isolation pads are typically formed of a solid elastomer. However, the classical isolation methods, due to their design of allowing the cryostat to freely displace without transferring vibration from the cryostat into a building structure, often lead to risk of increased rigid body motion of the cryostat, which has an adverse effect on image quality. Classical isolation methods use soft compliant materials that allow for a large amount of displacement.
It is desirable that a cryostat exhibit minimal rigid body motion generated from surrounding building structures. Building structures typically have a low resonating mechanical frequency or vibration that can transfer through solid compliant elastomeric pads or the like into a cryostat and amplify existing cryostat vibrations, which can result in poor image quality, and potentially render an MR imaging system ineffective in performing image reconstruction.
Use of soft elastomeric pads also adds risk to long-term performance due to slow changes in material properties therein over time also resulting in poor image quality. Over time the pads need to be replaced and the MR imaging system may need to be recalibrated, which is costly due to machine downtime, labor costs, and part replacement costs.
Moreover, there is a continuous need to update existing imaging systems. When updating an existing system various parameters may be altered or adjusted causing the resulting imaging system to generate a different set of vibrational frequencies then originally generated. Current isolation techniques do not account for system updates and imaging system and building structural responses generated therefrom.
It would therefore be desirable to provide a system or method of isolating a cryostat from a building structure that minimizes transfer of cryostat generated vibrations to proximate building structures, that exhibits minimal risk of long-term performance, that minimizes transfer of rigid body motion from a building structure, and accounts for system updates.
The present invention provides an apparatus and system for isolating mechanical vibrational disturbances between an MR imaging system and a surrounding structure. An MR system isolation impedance mismatch apparatus is provided that includes an impedance mismatch layer. The impedance mismatch layer performs as a mechanical notch filter and isolates an MR system component from a surrounding structure.
A method of tuning the impedance mismatch apparatus is also provided that includes determining a default notch filter frequency range. The impedance mismatch apparatus is formed and performs as a notch filter having the default notch filter frequency range. The impedance mismatch apparatus is installed and tested between the MR system component and the surrounding structure. Vibrations are detected in the MR system component and in the surrounding structure. The impedance mismatch apparatus is adjusted in response to the detected vibrations.
One of several advantages of the present invention is that it provides an impedance mismatch between an MR system component and a building structure. In so doing the present invention attenuates vibrational and acoustical noise generated from the MR system component and prevents amplification of vibrational energy contained within a building structure, thus preventing transfer of vibrational energy between the MR system component and the building structure. The present invention in minimizing transfer of vibrational energy between MR system components and building structures improves image quality and nearby working environments.
Another advantage of the present invention is that it utilizes relatively rigid materials that exhibit minimal long-term performance variations.
Furthermore, the present invention accounts for updates to an MR system by providing a method of tuning the impedance mismatch apparatus to adjust for the updates with minimal downtime and costs involved therein.
The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures.